A two-phase switched reluctance machine (TPSRM) is a switched reluctance machine (SRM) with two phases, phases A and B, displaced from each other by 180 degrees/n, where n is the number of stator poles per phase. FIGS. 7(a) and 7(b) illustrate a two-phase SRM 700 of the related art. TPSRM 700 has four stator poles 9-12 and two rotor poles 13, 14 and is characterized as a two-phase machine with a 4/2 stator/rotor pole combination (i.e., 4 stator poles and 2 rotor poles). The rotor rotates about a rotor shaft 15.
FIG. 7(a) illustrates TPSRM 700 and its flux paths through back iron segments 1-8, stator poles 9, 11, and rotor poles 13, 14 during the excitation of phase A. FIG. 7(b) illustrates TPSRM 700 and its flux paths through back iron segments 1-8, stator poles 10, 12, and rotor poles 13, 14 during the excitation of phase B. The phase A excitation is induced by winding 16, 16′ around stator pole 9 and another winding 17, 17′ on the diametrically opposed stator pole 11. Winding 16, 16′ and winding 17, 17′ may be connected in series or in parallel. The phase B excitation is induced by winding 18, 18′ around stator pole 10 and another winding 19, 19′ on the diametrically opposed stator pole 12. Winding 18, 18′ and winding 19, 19′ may similarly be connected in series or in parallel. In each of FIGS. 7(a) and 7(b), the rotor poles are positioned in alignment with the stator poles whose windings are being energized.
Excitation of the phase B windings is initiated when rotor pole 14 is positioned between stator poles 9 and 10 and rotor pole 13 is positioned between stator poles 11 and 12. The rate of change of inductance will be positive in this region and, hence, positive torque will be produced. As a result, rotor poles 14 and 13 will rotate clockwise (CW) toward the phase B stator poles 10 and 12, respectively. When rotor poles 14 and 13 pass around phase B stator poles 10 and 12, respectively, the phase A windings will be excited, causing rotor poles 14 and 13 to rotate CW toward the phase stator poles 11 and 9, respectively.
To reverse the direction of rotation, phase B windings 18, 18′ and 19, 19′ are excited when rotor pole 14 is between stator poles 12 and 9 and rotor pole 13 is between stator poles 10 and 11. If the rotor poles are closer to phase B stator poles 10 and 12, the rotor will rotate toward them resulting in counter-clockwise (CCW) rotation. Therefore, controlling the rotor's direction of rotation involves the independent control of the phase A and B excitations. If such independent control of individual phase excitation is not possible, there is no control method that can reverse the machine's rotor rotation from one direction to another.
Related art TPSRMs are driven by two or four controllable switch-based power converters. Some examples are described below.
FIG. 1 illustrates a related art asymmetric power converter 100 for driving a two-phase SRM. Power converter 100 has two controllable and two uncontrollable power devices for each phase winding 101, 102 of the SRM. Therefore, four controllable 103-106 and four uncontrollable 107-110 power devices are required for power converter 100 to operate. The primary advantage of power converter 100 is that it gives full controllability in terms of its ability to apply full positive or negative direct current (dc) link voltage, and, therefore, it does not diminish or restrict any operating mode of the SRM. The disadvantage of this power converter topology is that it uses eight power devices. A more detailed description of power converter 100's circuit operation may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001.
FIG. 2 illustrates a related art single switch-per-phase power converter 200 for driving a two-phase SRM. Power converter 200's circuit topology is based on splitting a dc input source voltage 201 equally to the machine side power converter. This results in a circuit requiring one controllable and one uncontrollable power device per phase winding 202, 203. Therefore, overall, power converter 200 requires two controllable power devices 204, 205 and two uncontrollable power devices 206, 207 for a two-phase SRM. The major advantage of this circuit design is that it uses a reduced number of power devices (e.g., a total of four) compared to the asymmetric converter. The disadvantage of this circuit is that it reduces the available dc source voltage by half and, therefore, doubles the current rating required for the devices and for the machine, resulting in low efficiency machine operation. A fuller description of this circuit may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001.
Some variations of the single-switch-per-phase topologies are described by Harris in U.S. Pat. No. 5,075,610, by Webster in U.S. Pat. No. 5,864,477, and by Disser et al. in U.S. Pat. No. 5,900,712. Harris describes a circuit with a single-switch-per-phase topology that requires more than one diode per phase and additionally requires more than one dc link capacitor. Also, the commutating voltages to the phase windings of Harris' power converter are always less than the source dc voltage in this circuit. These are severe limitations.
Webster and Disser each describe a single-switch-per-phase topology requiring two capacitors for storing energy in the circuit. Moreover, these topologies do not provide for a common emitter connection of the switches, thereby creating an isolation requirement for the gate drive circuits and a higher cost for the additional components. As a result of the above-mention and other factors, these circuits are strongly limited to low cost variable speed applications.
FIG. 3 illustrates a related art C-Dump power converter 300 for driving a two-phase SRM. Power converter 300's circuit uses three controllable power devices 301-303 and three uncontrollable diodes 304-306, resulting in the use of six power devices. This is an intermediate circuit between those illustrated in FIGS. 1 and 2. The operating modes are somewhat restricted for this circuit, since it can apply a full dc source voltage 309 to machine windings 307, 308 only in the positive direction. Furthermore, this circuit requires an external inductor 310 or a resistor (not shown) to dissipate the energy stored in a C-dump capacitor 311. Use of external inductor 310 increases the cost, whereas the use of the power resistor (not shown) will result in a lower efficiency of the system and higher package volume, due to increased thermal considerations. Therefore, this circuit is not ideal for use with two-phase SRMs. A more detailed description of this circuit may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001 and in Miller et al., U.S. Pat. No. 4,684,867, dated Aug. 4, 1987.
FIG. 4 illustrates a related art single switch-per-phase power converter 400 for driving a two-phase SRM. Power converter 400 requires one uncontrolled power device 401, 402 and one controlled power device 403, 404 per phase 405, 406, and therefore, requires four power devices to function. Furthermore, power converter 400 requires a special winding in the machine, known as a bifilar winding. This special winding increases the copper volume in the machine windings, resulting in increased cost for the machine. Additionally, power switches 403, 404 experience higher voltage stresses due to the leakage inductance between the windings of each respective phase. This leakage inductance can be minimized but cannot be eliminated in a practical machine. Therefore, this converter circuit is not widely used, despite the fact that a full dc source voltage 407 can be impressed on the machine with full controllability of the current. A more in depth description of this circuit may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001 and in Miller, U.S. Pat. No. 4,500,824, dated Feb. 19, 1985.
All other heretofore known two-phase power converter circuit topologies fall into one of the above-described categories, in terms of the total number of power devices required for their operation. From the foregoing, it may be seen that a minimum of two controllable power devices are required for operating a related art two-phase SRM.
Generally speaking though, commercial power converters used to drive a two-phase SRM usually have more than two controllable switches and more than two diodes. Circuits requiring only two controllable switches and two diodes have the disadvantages of high power loss, low efficiency, and a bifilar winding in the machine, thereby reducing the power density of the machine. Therefore, existing solutions are not attractive with regard to considerations of high efficiency operation, full range of speed control, compactness in the converter's packaging and, most importantly of all, the overall cost of the system.
A fundamental challenge in power converter development has been to reduce the number of power devices, both controllable and uncontrollable, to a level corresponding to that of a single-quadrant chopper drive, such as is commonly used in a dc motor drive or in a universal motor drive. A description of these drives is provided in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001. When the number of power devices has been reduced to this level, a brushless SRM drive becomes commercially competitive for variable speed applications. Moreover, the brushless SRM has the superior advantage of high efficiency, since there are no brushes and commutators in the SRM. Also, the brushless SRM is further endowed with high-speed operability, high reliability, maintenance-free operation, greater overload capability and, most of all, a cost advantage over the dc motor drive.
All reference material cited herein is hereby incorporated into this disclosure by reference.